Electric discharge device structure having separate heater and discharge compartments



Aug. 2, 19 66 J. E. BEGGS ELECTRIC DISCHARGE DEVICE STRUCTURE HAVING SEPARATE HEATER AND DISCHARGE GOMPARTMENTS Filed Nov. 21, 1962 Fig.

lnvenfor James E. 56995,

H/fs Attorney- United States Patent ELECTRIC DISCHARGE DEVICE STRUCTURE HAVING SEPARATE HEATER AND DIS- CHARGE CQMPARTMENTS James E. Beggs, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Nov. 21, 1962, Ser. No. 239,114 1 Claim. (Cl. 313-1) The present invention relates to improved electric discharge devices capable of operating at high emission density, particularly under high temperature Operating conditions.

In the design of electric discharge devices for any given application, certain of the operating conditions tend to impose conflicting requirements on the design. For higher power output larger cathode areas or higher emission densities may be employed. Larger cathode areas tend to conflict, however, with the smaller electrode size and spacings desired for high frequency operation. Higher emission densities may be attempted by higher operating temperatures of the cathode, but useful cathode life, particularly when the cathode is coated with alkaline earth oxide type of emission enhancing coating, places an upper limit on practical cathode operating temperatures. Even more discouraging is the failure to gain the expected increase in emission which is often experienced, particularly over a useful life of the device as the cathode temperature is increased. I have found that the failure to realize the emission levels hoped for is due, particularly in devices having oxide coated cathodes, to a contamination of the cathode coating by products evolved by parts of the device, such as the cathode heater, which operate at a higher temperature than the out-gassing temperature employed in the manufacture of the device. Even when high temperatures are reached during exhaust, it is not possible to maintain them for long enough intervals of time to completely degas the heater without damaging the cathode. Accordingly, it is an important object of my invention to provide an improved electric discharge device in which emission densities may be maintained at high levels during a'useful life of the device.

In accordance with a preferred embodiment of my invention, the high emission levels at high operating temperatures, particularly of oxide coated cathodes, is maintained over the life of the device by excluding the volume surrounding the heater from any communication with the volume enclosing the electrode surfaces which are active in the electric discharge. In the specific embodiment illustrated the cathode structure, including the cathode support and terminal, provides a gas tight barrier between the volume enclosing the heater on the one hand and the volume enclosing the anode, cathode and grid electrode surfaces on the other hand. Further objects and advantages characteristic of my invention will become apparent as the following description proceeds, reference being had to the accompanying drawing and its scope will be pointed out in the appended claim. In the drawing:

FIG. 1 is an elevational view in section of an electric discharge device embodying my invention; and

FIG. 2 is an elevational view in section showing a modification of my invention.

Referring now to FIG. 1 of the drawing, I have shown my invention embodied in a planar electrode type electric discharge device capable of high frequency, high temperature operation and capable of operating at high emission density which in turn provides high power output considering the operating frequencies. Referring now to the drawing, the envelope of the discharge device is made up of insulating spacers 10, 11 and 12 and anode, grid, cathode and heater metal terminal members 13, 14, 15

3,264,587 Patented August 2, 1966 and 16, respectively. Terminal members 13 and 16 are generally disk like and form the opposite end closure members of the generally cylindrical envelope, while the remaining terminals and spacers are of generally annular configuration. While materials for the terminals and spacers may be chosen in accordance with known considerations, it is particularly advantageous that the spacer members be of ceramic, such as alumina or forsterite and that the terminal members he of titanium. Such terminal members may be readily bonded to the ceramics in accordance with known processes, such as the one described in detail and claimed in Beggs Patent 2,857,663, dated October 28, 1958, assigned to the assignee of this invention. In accordance with the sealing method there described, a shim of metal is interposed between the metal terminal member and surface of the ceramic spacer at which the junction is to be made. Where the terminals are of titanium, these shims may be selected from the group consisting of nickel, copper, iron, cobalt or chromium. If the terminal member is one of the metals of the second group, such as copper, for example, then the sealing shim preferably is of titanium. The terminals 13, 14, and 15 provide support for an electrical connection with the anode, grid and cathode, respective-1y. In the particular embodiment illustrated, the anode is in the form of a tungsten cylinder 17 which is brazed to the inner surface of the central portion of the anode terminal member which as illustrated is of reduced thickness providing a recess within which is brazed a hollow copper cylinder 18 which provides for removal of heat from the anode and may be provided with a surrounding radiator, not shown. The control grid 19 includes a grid washer 20 which may to advantage be formed of tungsten having a plurality of closely spaced fine parallel grid wires 21 which may also be formed of tungsten to advantage and a plurality of orthogonally extending support wires 22 of larger diameter and preferably also of tungsten. The particular grid structure illustrated is described in more detail and claimed in US. Patent No. 3,238,411, filed concurrent-1y herewith and assigned to the assignee of this invention. As illustrated the grid washer 20 is positioned between the inwardly directed flange 23 on the grid terminal 14 and the upper surface of the gridcathode spacer 11.

The cathode structure 24 includes an inverted cup-like member 25 having an outwardly directed flange 26 at its lower end received in and bonded to the surfaces of a suitable recess in the cathode terminal 15. The circular and flattened end 27 of the cup-shaped member supports a disk-like cathode base or lid 28 which is bonded to the member 25 and provided on its upper surface with a coating 29 of electron emission enhancing material such as a mixture of alkaline earth carbonates which have been activated and reduced to oxides in the processing of the device during assembly and evacuation thereof. The cathode is heated to a temperature of copious electron emission by means of a generally cylindrical heater element 30 which may be a spiral tungsten wire having one terminal bonded to a cylindrical heat shield 31 having an outwardly directed flange 32 bonded to the lower side of the cup-shaped support 25. The other and coaxial terminal 33 of the heater extends through an opening 34 in the insulating spacer 12 and is joined to the center of the heater terminal and closure member 16. The heater element 30 is preferably coated with a thin coating of insulating material, such as alumina.

While the discharge device of the present invention may be assembled in different ways, it is of a construction which is particularly suited for what is known as a Bell jar assembly and exhaust. While this assembly technique is well known in the art, it may be described here briefly as involving supporting the stack of parts illusformed and the bond made upon cooling. The out gassing may take place at a temperature of about 900 C. to 950 C. when the materials used are ceramics and titanium, for example. A slightly higher temperature is used for forming the sealing eutectic of the metals of the shims and terminals. The cathode is also activated by heating during this process.

It will be noted also that the flange of the supporting 7 member of the cathode is hermetically sealed to the terminal 15 at this time sothat the portion of the envelope to which the heater element is exposed is hermetically sealed from the portion of the envelope enclosing the active surfaces of the grid, anode, and cathode- In this connection it is to be noted that the cathode member 25 is an integral cup, i.e., it is formed from a single piece of metal by a succession of deep drawing operations. While this unitary construction is a very desirable one from a standpoint of separating the envelope into two compartments hermetically sealed With respect to oneanother, it Will be readily appreciated that the gas-tight barrier may be fabricated from separate parts which are brazed together. In this way gases evolved during operation of the tube at high temperature do not reach the space between the anode and cathode and do not contaminate the cathode. Also, in the particular device described, the insulating parts of the envelope are of ceramic and a substantial area of metallic parts exposed to the interior of the device are of titanium. Both of these materials contribute to the operation of the device at high temperature with the titanium acting to sorb gases which tend to be released during the operation of the device and to contaminate the cathode.

It is to be noted that these same parts withstand high temperature processing during the evacuation, bake out and activation of'the cathode so that the tendency to liberate additional gases during operation is minimized.

In the device illustrated in the drawing, the heater may reach a temperature of the order of 1350" C. during op-' eration to maintain the emitting surface at a tempera ture of approximately 825 C. While the heater may be individually outgassed before assembly at a temperature of 1600l700 C., for example, it does not reach during assembly and evacuation of the tube an outgassing temperature as high as the 1350 C. at which it operates;

If it is raised to a higher temperature it is for so brief a time that itis not completely outgassed. Accordingly, there is a significant tendency for the heater to evolve some gas during operation and the present invention provides a structure which prevents these gases from reach-' ing the cathode and reducing the emission densities attainable. the necessity of extreme ca-re'in outgassing the heater which isnorm-ally one of the higher temperature parts in operation and tends to maintain high current densities uniformly from tube to tube independently of the gas evolving characteristics of the heater and to guarantee that these high current densities are obtained over a substantial operating life. With tubes of the type illustrated emission densities in the order of 2 amperes per square centimeter are obtained initially. Without the use 'of this invention, emission density may fall to a value onetenth the initial density in a matter of 30-40 hours if the heater is very gassy. In other tubes where the heater is more thoroughly outgassed, emission density may hold up for several hundred hours. however, assures uniformity of operation over a longer life and at current densities which are very high. It islto Viewed from another standpoint, it eliminates- The present invention, 1

be noted that these same current densities are not attainable over an operating lifewith ordinary low temperature parts such as ordinary glass envelopes. and the like since a high temperature-outgassing is'not possible. Accordingly, not only the compartmentalization of the: envelope separating the active elect-rode surfaces, from'the compartment housing the heater, but also the utilization of high temperature parts: which may be outgassed at high temperatures makes possible these devices having high current density capabilities over a commercial operat-v ing life.

The particular embodiment of my invention described above involves: aplanar electrode device. there is illustratedan elevational view of a device embodying my invention'and employing concentric cylindrical electrodes. As there illustrated the device may include a cup-shaped hollow cylindrical anode '35 which formsa major portion of the; vacuum tight envelope and is closed at its open end by an insulator and terminal assembly including; annular. insulator 36 bonded ,to the opened end of anode 35,?a circular, flange 37 providing av cathode terminal, and a cup-shaped'insulator '38 bonded to the terminal 37'and in cooperationwith the heater terminals 39 and 40 completing the envelope of the discharge device. The terminal 37, as. illustrated, provides acathode terminal and is formed integrally with a closed ended cylindrical-memberc38 I. which provides a support for a cylindrical cathode base member 39 to the exteriortsurface of which is applied:an electron;

emission enhancing material 40 consisting -of a mixture of alkaline earth carbonates. The member 38 provides not only the support for the cathode .but an impervious partition betweenithe portion of the envelope housing the anode and cathode or active electrodes and the .por-.

tion .42 housing 1 the heater element; 43 .which. is. illus-. trated schematically as a bifilarhelix having the endportions extending through openings 44 in the closure member 38 and connected respectively with the heater terminals 39 and 40.1 The; materials employed in the device of FIG. 2 may correspond .With those described in connection with the .embodimentof FIG. It will be apparent that the cathode support and terminal: structure. provides a gas impervious partitionsealed to theside walls of the device: :and separating the .interior of :the

envelope into two compartments which are gas; tight with respect to one another. In=a preferredform where the insulators 36 and 37 are of ceramic and the anode .of titanium,-the device is particularly suited forhigh operat-. ing. temperatures Where the separation of the heater compartment from the compartment housing theactive electrodes, is; particularly advantageous.

While I have described a particular embodiment of my. invention, it Will be apparent to those skilled in the art that changes and modificationsmay bemade without departing from my invention! in its broader aspects and I aim, therefore, in. the; appended. claim tocover all such changes and modifications as 'falLwithin [the .true

spiritland scope of my invention.

What I claim as new and desire to secure by Letters Patent of the. United States is:

Anpelectric discharge device'comprising alternately;

connected to a spaced'and mutually insulated one of said 1 metal terminals, said anode presenting a continuous electron collecting surface in opposed relation to the emitting surface of said cathode, a resistance heater elementhava ing an'insulating oxide coating thereon supported within said envelope for heating said cathode and located with .in saidenvelo'pe .on the side. of said. cathode remote In FIG. 2

from said anode, and means extending transversely of said envelope and joined to said one of said metal members and said cathode to divide the interior thereof into two compartments which are vacuum tight with respect to one another, one of said compartments enclosing said resistance heater element, the other of said compartments enclosing the opposed surfaces of said anode and cathode so that gases liberated from said means for heating are excluded from the region of said cathode and operation of said heater at temperatures above the temperature at which the device is outgased during evacuation does not contaminate said cathode surface.

References Cited by the Examiner UNITED STATES PATENTS 2,932,755 4/1960 Jeppson 3 13-3 37 X 2,953,701 9/1960 Gale 313346 X 3,114,068 10/ 1963 Shortridge 313-237 3,119,041 1/1964 Harris 313-346 X JOHN W. HUCKERT, Primary Examiner.

10 DAVID J. GALVIN, Examiner.

L. ZALMAN, Assistant Examiner. 

